14 N NQR STUDIES OF STABILIZERS DPA, METHYL- AND ETHYL-CENTRALITE J. Luznik, J. Pirnat T. Apih and Z. Trontelj Institute of Mathematics, Physics and Mechanics and Institute J. Stefan, University of Ljubljana, Slovenia. Introduction
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14N NQR STUDIES OF STABILIZERS DPA, METHYL- AND ETHYL-CENTRALITE
J. Luznik, J. Pirnat T. Apih and Z. Trontelj
Institute of Mathematics, Physics and Mechanics and Institute J. Stefan, University of Ljubljana, Slovenia
Nuclear Quadrupole Resonance (NQR) with its ability of identification specific molecules in measured sample is potentially powerful method in solid state physics, chemistry and pharmacy. Namely, molecules and atoms with quadrupole nuclei are in each chemical compound in different electric environment, hence they are characterized with different NQR transition frequencies.
Nitrogen nuclei are present in many organic chemical compounds and thus enable their nitrogen NQR spectroscopy studies. 14N NQR transition frequencies for different substances are covering a broad frequency range from around 100 kHz to about 5 MHz. Unfortunately the nitrogen resonance frequencies of some most interesting chemical compounds are found at the very low frequencies. Hence, their detection is difficult because of the very poor signal-to-noise (S/N) ratio. The required signal averaging and measuring times are therefore often of the order of several hours and thus too long for practical applications.
With the recently improved techniques (proton-nitrogen level crossing polarization transfer combined with proper pulsed spin-locking sequencies) the NQR of very low frequency 14N lines in some energetic materials and explosives became interesting for different applications. We will demonstrate some results of 14N NQR signal temperature dependence and correlation between 14N NQR signal intensity and concentration studies of stabilizers* Diphenylamine, Methyl- and Ethyl-centralite. This kind of 14N NQR application is promissing for studies of quality, stability and degradation of explosives and propellants.
Figure2: Temperature dependence of 14N NQR in Diphenylamine
14N NQRinvestigations of three widely used stabilizers Diphenylamine, Methylcentralite and Ethylcentralite were performed. Their chemical furmulae and molecular structures are shown on Fig. 1. 14N NQR signals were measured from room temperature to 77 K in all three samples. Temperature dependences of NQR transition frequencies in all three samples are similar and do not show any irregularity. Fig. 2 shows the example of NQR temperature dependence in Diphenylamine. Approximately 10 g of samples were used in a solenoidal coil with 16 mm diameter. Very strong and easily detectable 14N NQR signals were obtained within the whole measured temperature interval. Some problems were with Diphenylamine at higher temperature (around room temperature) where the signal intensity in this sample is reduced. In our experiments the multipulse pulse spin-locking (PSL) sequence was applied: a0 - ( t - a90- t - )n, where n refers to the number of pulse-train repetitions and the pulse width a is chosen to optimize the signal.Fig.3 shows a typical signal of the n+ transition in Methylcentralite obtained in a “single shot” experiment with averaging 100 echos within one PSL sequence.
Figure3: 14N NQR signal at 3768 kHz in Methylcentralite at room temperature
In all three samples we were able to get very good 14N NQR spectra. The NQR transition frequencies for them are collected in Table 1. The numbers of different frequencies (corresponding to different positions of nitrogen atoms in crystal unit cell) for all the samples are in agreement with the known crystal structures [1,2].
Table 1: NQR frequencies in DPA, MC and EC
*Stabilizers are added to explosives and propellants in small amounts mostly less than 5 %. To follow the aging and degradation of explosives and propellants accurate determination of these low concentrations is important. We tried to determine the accuracy of such measurement with diluted samples of Methylcentralite down to 0,5 % (Fig. 4).
Figure 1: Structural and chemical formulae of tested stabilizers
 Mark A. Rodriguez and Scott D. Bunge, Acta Cryst. E59, o1123-o1125 (2003)
 P. Ganis, G. Avitabile, E. Benedetti, C. Pedone, and M. Goodman, Proceedings of the National Academy of Sciences, Vol. 67, No. 1, pp. 426-433, September 1970
Figure 4: Intensity of the NQR signal in Methylcentralite versus concentration